Hello there, svbapatandco.

I'd like to help you with your concern. When you say post a unique document no. are you referring to the custom transaction numbers? If so, you can enable the option by following these steps:

1. Go to the Gear > Account and settings.
2. On the left panel, click Sales.
3. In the Sales form content section, click the toggle icon to turn on Customer Transaction Numbers.
4. Hit Save and then Done. 

Here's a sample screenshot for a visual guide:

You can also read this article for reference: How to show transaction numbers on sales forms.

If you're referring to something else, please feel free to add more details in your reply. I'll be right here to help you out again. Thanks.